input: don't modify input->stream counters from seqsrc internals

ideally the input->stream->count_recv counter should be retrievable
without seqsrc tracking enabled, so increment count_recv in
_input_nmsg_read_container_sock() instead.

also, it's not necessary for the seqsrc tracking code to be responsible
for updating input->stream->count_drop. instead, return the number of
dropped containers from _input_seqsrc_update() and do the update of
input->stream->count_recv from _input_nmsg_read_container_sock().  this
keeps all the input->stream counter updates in the same function.